Martensitic stainless steel seamless pipe for oil country tubular goods, and method for manufacturing same

ABSTRACT

The invention provides a martensitic stainless steel seamless pipe for oil country tubular goods having a yield stress of 758 MPa (110 ksi) or more, and excellent sulfide stress corrosion cracking resistance and a method for manufacturing the same. The martensitic stainless steel seamless pipe for oil country tubular goods having a yield stress of 758 MPa or more has a composition that contains, in mass %, C: 0.010% or more, Si: 0.5% or less, Mn: 0.05 to 0.24%, P: 0.030% or less, S: 0.005% or less, Ni: 4.6 to 8.0%, Cr: 10.0 to 14.0%, Mo: 1.0 to 2.7%, Al: 0.1% or less, V: 0.005 to 0.2%, N: 0.1% or less, Ti: 0.06 to 0.25%, Cu: 0.01 to 1.0%, and Co: 0.01 to 1.0%, in which C, Mn, Cr, Cu, Ni, Mo, W, Nb, N, and Ti satisfy the predetermined relations, and the balance is Fe and incidental impurities.

CROSS REFERENCE TO RELATED APPLICATIONS

This is the U.S. National Phase application of PCT/JP2018/032685, filedSep. 4, 2018, which claims priority to Japanese Patent Application No.2017-190075, filed Sep. 29, 2017, the disclosures of each of theseapplications being incorporated herein by reference in their entiretiesfor all purposes.

FIELD OF THE INVENTION

The present invention relates to a martensitic stainless steel seamlesspipe for oil country tubular goods for use in crude oil well and naturalgas well applications (hereinafter, referred to simply as “oil countrytubular goods”), and to a method for manufacturing such a martensiticstainless steel seamless pipe. Particularly, the invention relates to aseamless steel pipe for oil country tubular goods having a yield stressYS of 758 MPa or more, and excellent sulfide stress corrosion crackingresistance (SSC resistance) in a hydrogen sulfide (H₂S)-containingenvironment, and to a method for manufacturing such a seamless steelpipe.

BACKGROUND OF THE INVENTION

Increasing crude oil prices and an expected shortage of petroleumresources in the near future have prompted active development of oilcountry tubular goods for use in applications that were unthinkable inthe past, for example, such as in deep oil fields, and in oil fields andgas oil fields of severe corrosive environments containing carbondioxide gas, chlorine ions, and hydrogen sulfide. The material of steelpipes for oil country tubular goods intended for these environmentsrequires high strength, and excellent corrosion resistance.

Oil country tubular goods used for mining of oil fields and gas fieldsof an environment containing carbon dioxide gas, chlorine ions, and thelike typically use 13% Cr martensitic stainless steel pipes. There hasalso been global development of oil fields in very severe corrosiveenvironments containing hydrogen sulfide. Accordingly, the need for SSCresistance is high, and there has been increasing use of an improved 13%Cr martensitic stainless steel pipe of a reduced C content and increasedNi and Mo contents.

PTL 1 describes a 13% Cr-base martensitic stainless steel pipe of acomposition containing carbon in an ultra low content of 0.015% or less,and 0.03% or more of Ti. It is stated in PTL 1 that this stainless steelpipe has high strength with a yield stress on the order of 95 ksi, lowhardness with an HRC of less than 27, and excellent SSC resistance. PTL2 describes a martensitic stainless steel that satisfies 6.0≤Ti/C≤10.1,based on the finding that Ti/C has a correlation with a value obtainedby subtracting a yield stress from a tensile stress. It is stated in PTL2 that this technique, with a value of 20.7 MPa or more yielded as thedifference between tensile stress and yield stress, can reduce hardnessvariation that impairs SSC resistance.

PTL 3 describes a martensitic stainless steel containing Mo in a limitedcontent of Mo≥2.3−0.89Si+32.2C, and having a metal microstructurecomposed mainly of tempered martensite, carbides that have precipitatedduring tempering, and intermetallic compounds such as a Laves phase anda δ phase formed as fine precipitates during tempering. It is stated inPTL 3 that the steel produced by this technique has high strength with a0.2% proof stress of 860 MPa or more, and excellent carbon dioxidecorrosion resistance and sulfide stress corrosion cracking resistance.

PATENT LITERATURE

PTL 1: JP-A-2010-242163

PTL 2: WO2008/023702

PTL 3: WO2004/057050

SUMMARY OF THE INVENTION

The development of recent oil fields and gas fields is made in severecorrosive environments containing CO₂, Cl⁻, and H₂S. Increasing H₂Sconcentrations due to aging of oil fields and gas fields are also ofconcern. Steel pipes for oil country tubular goods for use in theseenvironments are therefore required to have excellent sulfide stresscorrosion cracking resistance (SSC resistance).

PTL 1 states that sulfide stress cracking resistance can be maintainedunder an applied stress of 655 MPa in an atmosphere of a 5% NaCl aqueoussolution (H₂S: 0.10 bar) having an adjusted pH of 3.5. The steeldescribed in PTL 2 has sulfide stress cracking resistance in anatmosphere of a 20% NaCl aqueous solution (H₂S: 0.03 bar, CO₂ bal.)having an adjusted pH of 4.5. The steel described in PTL 3 has sulfidestress cracking resistance in an atmosphere of a 25% NaCl aqueoussolution (H₂S: 0.03 bar, CO₂ bal.) having an adjusted pH of 4.0.However, these patent applications do not take into account sulfidestress corrosion cracking resistance in other atmospheres, and it cannotbe said that the steels described in these patent applications have thelevel of sulfide stress corrosion cracking resistance that can withstandthe today's ever demanding severe corrosive environments.

It is accordingly an object of the present invention to provide amartensitic stainless steel seamless pipe for oil country tubular goodshaving a yield stress of 758 MPa (110 ksi) or more, and excellentsulfide stress corrosion cracking resistance. The invention is alsointended to provide a method for manufacturing such a martensiticstainless steel seamless pipe.

As used herein, “excellent sulfide stress corrosion cracking resistance”means that a test piece dipped in a test solution (a 0.165 mass % NaClaqueous solution; liquid temperature: 25° C.; H₂S: 1 bar; CO₂ bal.)having an adjusted pH of 3.5 with addition of sodium acetate andhydrochloric acid, and in a test solution (a 20 mass % NaCl aqueoussolution; liquid temperature: 25° C.; H₂S: 0.1 bar; CO₂ bal.) having anadjusted pH of 5.0 with addition of 0.82 g/L of sodium acetate andacetic acid does not crack even after 720 hours under an applied stressequal to 90% of the yield stress.

In order to achieve the foregoing objects, the present inventorsconducted intensive studies of the effects of various alloy elements onsulfide stress corrosion cracking resistance (SSC resistance) in a CO₂,Cl⁻-, and H₂S-containing corrosive environment, using a 13% Cr-basestainless steel pipe as a basic composition. The studies found that amartensitic stainless steel seamless pipe for oil country tubular goodshaving the desired strength, and excellent SSC resistance in a CO₂,Cl⁻-, and H₂S-containing corrosive environment, and in an environmentunder an applied stress close to the yield stress can be provided whenthe steel has a composition in which the components are contained inpredetermined ranges, and C, Mn, Cr, Cu, Ni, Mo, W, Nb, N, and Ti arecontained in adjusted amounts that satisfy appropriate relations andranges, and when the steel is subjected to appropriate quenching andtempering.

The present invention is based on this finding, and was completed afterfurther studies. Specifically, the gist of the exemplary embodiments ofthe present invention is as follows.

[1] A martensitic stainless steel seamless pipe for oil country tubulargoods having a yield stress of 758 MPa or more,

the martensitic stainless steel seamless pipe comprising, in mass %, C:0.010% or more, Si: 0.5% or less, Mn: 0.05 to 0.24%, P: 0.030% or less,S: 0.005% or less, Ni: 4.6 to 8.0%, Cr: 10.0 to 14.0%, Mo: 1.0 to 2.7%,Al: 0.1% or less, V: 0.005 to 0.2%, N: 0.1% or less, Ti: 0.06 to 0.25%,Cu: 0.01 to 1.0%, and Co: 0.01 to 1.0%, in which the values of thefollowing formulae (1), (2), and (3) satisfy the formulae (4) below, andthe balance is Fe and incidental impurities.

−109.37C+7.307Mn+6.399Cr+6.329Cu+11.343Ni−13.529Mo+1.276W+2.925Nb+196.775N−2.621Ti−120.307  Formula(1)

−0.0278Mn+0.0892Cr+0.00567Ni+0.153Mo−0.0219W−1.984N+0.208Ti−1.83  Formula(2)

−1.324C+0.0533Mn+0.0268Cr+0.0893Cu+0.00526Ni+0.0222Mo−0.0132W−0.473N−0.5Ti−0.514  Formula(3)

In the formulae, C, Mn, Cr, Cu, Ni, Mo, W, Nb, N, and Ti represent thecontent of each element in mass %, and the content is 0 (zero) forelements that are not contained.

−35.0≤value of formula (1)≤45.0,

−0.600≤value of formula (2)≤−0.250, and

−0.400≤value of formula (3)≤0.100

[2] The martensitic stainless steel seamless pipe for oil countrytubular goods according to item [1], wherein the composition furthercomprises, in mass %, at least one selected from Nb: 0.1% or less, andW: 1.0% or less.

[3] The martensitic stainless steel seamless pipe for oil countrytubular goods according to item [1] or [2], wherein the compositionfurther comprises, in mass %, one or more selected from Ca: 0.010% orless, REM: 0.010% or less, Mg: 0.010% or less, and B: 0.010% or less.

[4] A method for manufacturing a martensitic stainless steel seamlesspipe for oil country tubular goods,

the method comprising:

forming a steel pipe from a steel pipe material of the composition ofany one of items [1] to [3];

quenching the steel pipe by heating the steel pipe to a temperatureequal to or greater than an Ac₃ transformation point, and cooling thesteel pipe to a cooling stop temperature of 100° C. or less; and

tempering the steel pipe at a temperature equal to or less than an Ac₁transformation point.

The exemplary embodiments of present invention has enabled production ofa martensitic stainless steel seamless pipe for oil country tubulargoods having excellent sulfide stress corrosion cracking resistance (SSCresistance) in a CO₂, Cl⁻-, and H₂S-containing corrosive environment,and high strength with a yield stress YS of 758 MPa (110 ksi) or more.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

The following describes the reasons for specifying the composition of asteel pipe of the present invention. In the following, “%” means percentby mass, unless otherwise specifically stated.

C: 0.010% or More

C is an important element involved in the strength of the martensiticstainless steel, and is effective at improving strength. In anembodiment of the present invention, the C content is limited to 0.010%or more to provide the desired strength. When contained in excessivelylarge amounts, C increases hardness, and susceptibility to sulfidestress corrosion cracking increases. For this reason, C is contained inan amount of desirably 0.040% or less. The preferred C content istherefore 0.010 to 0.040%.

Si: 0.5% or Less

Si acts as a deoxidizing agent, and is contained in an amount ofdesirably 0.05% or more. A Si content of more than 0.5% impairs carbondioxide corrosion resistance and hot workability. For this reason, theSi content is limited to 0.5% or less. From the viewpoint of stablyproviding strength, the Si content is preferably 0.10 to 0.30%.

Mn: 0.05 to 0.24%

Mn is an element that improves hot workability and strength. Mn iscontained in an amount of desirably 0.05% or more to provide thenecessary strength. When contained in an amount of more than 0.24%, Mngenerates large amounts of MnS as inclusions. This becomes an initiationpoint of pitting corrosion, and impairs the sulfide stress corrosioncracking resistance. For this reason, the Mn content is limited to 0.05to 0.24%.

P: 0.030% or Less

P is an element that impairs carbon dioxide corrosion resistance,pitting corrosion resistance, and sulfide stress corrosion crackingresistance, and should desirably be contained in as small an amount aspossible in the present invention. However, an excessively small Pcontent increases the manufacturing cost. For this reason, the P contentis limited to 0.030% or less, which is a content range that does notcause a severe impairment of characteristics, and that is economicallypractical in industrial applications. Preferably, the P content is0.015% or less.

S: 0.005% or Less

S is an element that seriously impairs hot workability, and shoulddesirably be contained in as small an amount as possible. A reduced Scontent of 0.005% or less enables pipe production using an ordinaryprocess, and the S content is limited to 0.005% or less in an embodimentof the present invention. Preferably, the S content is 0.002% or less.

Ni: 4.6 to 8.0%

Ni is an element that increases the strength of the protective coating,and improves the corrosion resistance. Ni also increases steel strengthby forming a solid solution. Ni needs to be contained in an amount of4.6% or more to obtain these effects. With a Ni content of more than8.0%, the martensite phase becomes less stable, and the strengthdecreases. For this reason, the Ni content is limited to 4.6 to 8.0%.

Cr: 10.0 to 14.0%

Cr is an element that forms a protective coating, and improves thecorrosion resistance. The required corrosion resistance for oil countrytubular goods can be provided when Cr is contained in an amount of 10.0%or more. A Cr content of more than 14.0% facilitates ferrite generation,and a stable martensite phase cannot be provided. For this reason, theCr content is limited to 10.0 to 14.0%. Preferably, the Cr content is11.0 to 13.5%.

Mo: 1.0 to 2.7%

Mo is an element that improves the resistance against pitting corrosionby Cl⁻. Mo needs to be contained in an amount of 1.0% or more to obtainthe corrosion resistance necessary for a severe corrosive environment.Mo is also an expensive element, and a Mo content of more than 2.7%increases the manufacturing cost. For this reason, the Mo content islimited to 1.0 to 2.7%. Preferably, the Mo content is 1.5 to 2.5%.

Al: 0.1% or Less

Al acts as a deoxidizing agent, and an Al content of 0.01% or more isneeded to obtain this effect. However, Al has an adverse effect ontoughness when contained in an amount of more than 0.1%. For thisreason, the Al content is limited to 0.1% or less in an embodiment ofthe present invention. Preferably, the Al content is 0.01 to 0.03%.

V: 0.005 to 0.2%

V needs to be contained in an amount of 0.005% or more to improve steelstrength through precipitation hardening, and to improve sulfide stresscorrosion cracking resistance. Because a V content of more than 0.2%impairs toughness, the V content is limited to 0.005 to 0.2% in anembodiment of the present invention.

N: 0.1% or Less

N acts to improve pitting corrosion resistance, and to increase strengthby forming a solid solution in the steel. However, N forms variousnitride inclusions in large amounts, and impairs pitting corrosionresistance when contained in an amount of more than 0.1%. For thisreason, the N content is limited to 0.1% or less in an embodiment of thepresent invention. Preferably, the N content is 0.010% or less.

Ti: 0.06 to 0.25%

When contained in an amount of 0.06% or more, Ti forms carbides, and canreduce hardness by reducing solid-solution carbon. However, the Ticontent is limited to 0.06 to 0.25% because, when contained in an amountof more than 0.25%, Ti generates TiN as inclusions, and impairs thesulfide stress corrosion cracking resistance as it becomes an initiationpoint of pitting corrosion. The Ti content is preferably 0.08 to 0.15%.

Cu: 0.01 to 1.0%

Cu is contained in an amount of 0.01% or more to increase the strengthof the protective coating, and improve the sulfide stress corrosioncracking resistance. However, when contained in an amount of more than1.0%, Cu precipitates into CuS, and impairs hot workability. For thisreason, the Cu content is limited to 0.01 to 1.0%.

Co: 0.01 to 1.0%

Co is an element that reduces hardness, and improves pitting corrosionresistance by raising the Ms point, and promoting a transformation. Coneeds to be contained in an amount of 0.01% or more to obtain theseeffects. When contained in excessively large amounts, Co may impairtoughness, and increases the material cost. For this reason, the Cocontent is limited to 0.01 to 1.0% in an embodiment of the presentinvention. The Co content is more preferably 0.03 to 0.6%.

In an embodiment of the present invention, C, Mn, Cr, Cu, Ni, Mo, W, Nb,N, and Ti are contained so that the values of the following formulae(1), (2), and (3) satisfy the formulae (4) below. Formula (1) correlatesthese elements with an amount of retained γ. By making the value offormula (1) smaller, the retained austenite occurs in smaller amounts,the hardness decreases, and the sulfide stress corrosion crackingresistance improves. Formula (2) correlates the elements withrepassivation potential. When C, Mn, Cr, Cu, Ni, Mo, W, Nb, N, and Tiare contained so that the value of formula (1) satisfies the range offormula (4), and Mn, Cr, Ni, Mo, W, N, and Ti are contained so that thevalue of formula (2) satisfies the range of formula (4), the passivationfilm can more easily regenerate, and the repassivation improves. Formula(3) correlates the elements with pitting corrosion potential. When C,Mn, Cr, Cu, Ni, Mo, W, Nb, N, and Ti are contained so that the value offormula (1) satisfies the range of formula (4), and C, Mn, Cr, Cu, Ni,Mo, W, N, and Ti are contained so that the value of formula (3)satisfies the range of formula (4), generation of pitting corrosion,which becomes an initiation point of sulfide stress corrosion cracking,can be reduced, and the sulfide stress corrosion cracking resistancegreatly improves. When the value of formula (1) satisfies the range offormula (4), the hardness increases when the value of formula (1) is 10or more. However, it is still possible to regenerate a passivation film,and to effectively reduce generation of pitting corrosion and improvesulfide stress corrosion cracking resistance when the value of formula(2) or (3) satisfies the range of formula (4).

−109.37C+7.307Mn+6.399Cr+6.329Cu+11.343Ni−13.529Mo+1.276W+2.925Nb+196.775N−2.621Ti−120.307  Formula(1)

−0.0278Mn+0.0892Cr+0.00567Ni+0.153Mo−0.0219W−1.984N+0.208Ti−1.83  Formula(2)

−1.324C+0.0533Mn+0.0268Cr+0.0893Cu+0.00526Ni+0.0222Mo−0.0132W−0.473N−0.5Ti−0.514  Formula(3)

In the formulae, C, Mn, Cr, Cu, Ni, Mo, W, Nb, N, and Ti represent thecontent of each element in mass %, and the content is 0 (zero) forelements that are not contained.

Formulae (4)

−35.0≤value of formula (1)≤45.0,

−0.600≤value of formula (2)≤−0.250, and

−0.400≤value of formula (3)≤0.100

At least one selected from Nb: 0.1% or less, and W: 1.0% or less may becontained as optional elements, as needed.

Nb forms carbides, and can reduce hardness by reducing solid-solutioncarbon. However, Nb may impair toughness when contained in anexcessively large amount. W is an element that improves pittingcorrosion resistance. However, W may impair toughness, and increases thematerial cost when contained in an excessively large amount. For thisreason, Nb and W, when contained, are contained in limited amounts ofNb: 0.1% or less, and W: 1.0% or less.

One or more selected from Ca: 0.010% or less, REM: 0.010% or less, Mg:0.010% or less, and B: 0.010% or less may be contained as optionalelements, as needed.

Ca, REM, Mg, and B are elements that improve corrosion resistance bycontrolling the form of inclusions. The desired contents for providingthis effect are Ca: 0.0005% or more, REM: 0.0005% or more, Mg: 0.0005%or more, and B: 0.0005% or more. Ca, REM, Mg, and B impair toughness andcarbon dioxide corrosion resistance when contained in amounts of morethan Ca: 0.010%, REM: 0.010%, Mg: 0.010%, and B: 0.010%. For thisreason, the contents of Ca, REM, Mg, and B, when contained, are limitedto Ca: 0.010% or less, REM: 0.010% or less, Mg: 0.010% or less, and B:0.010% or less.

The balance is Fe and incidental impurities in the composition.

The following describes a preferred method for manufacturing a stainlesssteel seamless pipe for oil country tubular goods of the presentinvention.

In the present invention, a steel pipe material of the foregoingcomposition is used. However, the method of production of a stainlesssteel seamless pipe used as a steel pipe material is not particularlylimited, and any known seamless pipe manufacturing method may be used.

Preferably, a molten steel of the foregoing composition is made intosteel using an ordinary steel making process such as by using aconverter, and formed into a steel pipe material, for example, a billet,using a method such as continuous casting, or ingot casting-blooming.The steel pipe material is then heated, and hot worked into a pipe usinga known pipe manufacturing process, for example, the Mannesmann-plugmill process, or the Mannesmann-mandrel mill process to produce aseamless steel pipe of the foregoing composition.

The process after the production of the steel pipe from the steel pipematerial is not particularly limited. Preferably, the steel pipe issubjected to quenching in which the steel pipe is heated to atemperature equal to or greater than an Ac₃ transformation point, andcooled to a cooling stop temperature of 100° C. or less, followed bytempering at a temperature equal to or less than an Ac₁ transformationpoint.

Quenching

In an embodiment of the present invention, the steel pipe is subjectedto quenching, in which the steel pipe is reheated to a temperature equalto or greater than an Ac₃ transformation point, held for preferably atleast 5 min, and cooled to a cooling stop temperature of 100° C. orless. This makes it possible to produce a refined, tough martensitephase. When the quenching heating temperature is less than an Ac₃transformation point, the microstructure does not occur in the austenitesingle-phase region, and a sufficient martensite microstructure does notoccur in the subsequent cooling, with the result that the desired highstrength cannot be obtained. For this reason, the quenching heatingtemperature is limited to a temperature equal to or greater than an Ac₃transformation point. The cooling method is not limited. Typically, thesteel pipe is air cooled (at a cooling rate of 0.05° C./s or more and20° C./s or less) or water cooled (at a cooling rate of 5° C./s or moreand 100° C./s or less), and the cooling rate conditions are not limitedeither.

Tempering

The quenched steel pipe is tempered. The tempering is a process in whichthe steel pipe is heated to a temperature equal to or less than an Ac₁transformation point, held for preferably at least 10 min, and aircooled. When the tempering temperature is higher than an Ac₁transformation point, the martensite phase precipitates after thetempering, and the desired high toughness and excellent corrosionresistance cannot be provided. For this reason, the temperingtemperature is limited to a temperature equal to or less than an Ac₁transformation point. The Ac₃ transformation point (° C.) and the Ac₁transformation point (° C.) can be determined by giving a heating andcooling temperature history to a test piece, and finding atransformation point from a microdisplacement due to expansion andcontraction in a Formaster test.

EXAMPLES

The present invention is further described below through Examples.

Molten steels containing the components shown in Table 1 were made intosteel with a converter, and cast into billets (steel pipe material) bycontinuous casting. The billet was hot worked into a pipe with a modelseamless rolling mill, and cooled by air cooling or water cooling toproduce a seamless steel pipe measuring 83.8 mm in outer diameter and12.7 mm in wall thickness.

Each seamless steel pipe was cut to obtain a test material, which wasthen subjected to quenching and tempering under the conditions shown inTable 2. A test piece for microstructure observation was taken from thequenched and tempered test material. After polishing, the amount ofretained austenite (γ) was measured by X-ray diffractometry.

Specifically, the amount of retained austenite was found by measuringthe diffraction X-ray integral intensities of the γ (220) plane and theα (211) plane. The results were then converted using the followingequation.

γ(volume fraction)=100/(1+(I_(α)R_(γ)/I_(γ)R_(α)))

In the equation, I_(α) represents the integral intensity of α, R_(α)represents a crystallographic theoretical calculation value for α, I_(γ)represents the integral intensity of γ, and R_(γ) represents acrystallographic theoretical calculation value for γ.

An arc-shaped tensile test specimen specified by API standard was takenfrom the quenched and tempered test material, and the tensile properties(yield stress, YS; tensile stress, TS) were determined in a tensile testconducted according to the API specification. The Ac₃ point (° C.) andAc₁ point (° C.) in Table 2 were measured in a Formaster test using atest piece (4 mmϕ×10 mm) taken from the quenched test material.Specifically, the test piece was heated to 500° C. at 5° C./s, and to920° C. at 0.25° C./s. After being held for 10 minutes, and test piecewas cooled to room temperature at a rate of 2° C./s. The expansion andcontraction of the test piece with this temperature history were thendetected to obtain the Ac₃ point (° C.) and Ac₁ point (° C.).

The SSC test was conducted according to NACE TM0177, Method A. A testenvironment was created by adjusting the pH of a test solution (a 0.165mass % NaCl aqueous solution; liquid temperature: 25° C.; H₂S: 1 bar;CO₂ bal.) to 3.5 with addition of sodium acetate and acetic acid (testenvironment 1), and by adjusting the pH of a test solution (a 20 mass %NaCl aqueous solution; liquid temperature: 25° C.; H₂S: 0.1 bar; CO₂bal.) to 5.0 with addition of 0.82 g/L of sodium acetate and acetic acid(test environment 2), and a stress 90% of the yield stress was appliedfor 720 hours in the solutions. Samples were determined as beingacceptable when there was no crack in the test piece after the test, andunacceptable when the test piece had a crack after the test.

The results are presented in Table 2.

TABLE 1 Steel Composition (mass %) No. C Si Mn P S Ni Cr Mo Al V N Ti CuA 0.0106 0.20 0.21 0.015 0.001 5.91 12.0 1.86 0.040 0.044 0.0045 0.0920.21 B 0.0112 0.19 0.21 0.017 0.001 5.98 12.4 2.20 0.042 0.014 0.00650.088 0.07 C 0.0109 0.20 0.19 0.015 0.001 5.84 11.9 2.02 0.044 0.0380.0078 0.101 0.32 D 0.0116 0.19 0.20 0.015 0.001 5.71 11.9 1.97 0.0420.044 0.0048 0.106 0.14 E 0.0125 0.21 0.21 0.014 0.001 4.61 12.5 1.840.039 0.023 0.0054 0.097 0.31 F 0.0104 0.17 0.21 0.014 0.001 6.12 11.92.69 0.039 0.024 0.0152 0.082 0.56 G 0.0154 0.20 0.12 0.015 0.001 7.2312.7 2.26 0.040 0.014 0.0048 0.152 0.51 H 0.0113 0.19 0.18 0.014 0.0016.34 12.5 2.18 0.039 0.037 0.0065 0.143 0.34 I 0.0161 0.20 0.07 0.0140.001 5.23 12.8 1.78 0.041 0.013 0.0074 0.084 0.46 J 0.0103 0.19 0.230.015 0.001 6.84 13.1 2.22 0.039 0.044 0.0084 0.113 0.21 K 0.0094 0.190.21 0.015 0.001 5.81 11.8 1.92 0.040 0.015 0.0103 0.098 0.34 L 0.01080.17 0.25 0.015 0.001 6.08 12.0 2.61 0.041 0.025 0.0143 0.072 0.54 M0.0131 0.18 0.11 0.014 0.001 4.52 13.9 1.36 0.039 0.028 0.0063 0.1210.54 N 0.0104 0.19 0.22 0.015 0.001 6.29 12.3 2.81 0.039 0.044 0.00740.108 0.18 O 0.0148 0.20 0.08 0.014 0.001 5.27 12.9 1.69 0.041 0.0330.0038 0.054 0.43 P 0.0115 0.18 0.20 0.015 0.001 5.66 11.8 1.94 0.0420.028 0.0053 0.108 1.08 Q 0.0109 0.17 0.23 0.014 0.001 6.18 11.7 2.680.039 0.015 0.0134 0.065 0.69 R 0.0104 0.19 0.23 0.015 0.001 7.82 13.81.23 0.041 0.009 0.0150 0.069 0.93 S 0.0487 0.20 0.07 0.014 0.001 4.7811.0 2.66 0.042 0.013 0.0038 0.211 0.02 T 0.0479 0.20 0.08 0.015 0.0017.84 13.8 2.65 0.040 0.014 0.0048 0.210 0.91 U 0.0112 0.19 0.23 0.0150.001 4.69 11.1 1.21 0.040 0.042 0.0154 0.072 0.03 V 0.0100 0.19 0.240.015 0.001 7.42 13.3 2.27 0.040 0.015 0.0023 0.061 2.31 W 0.0101 0.190.06 0.013 0.001 4.62 10.7 2.02 0.041 0.042 0.0546 0.405 0.01 ValueValue Value of of of formula formula formula Steel Composition (mass %)(1) (2) (3) No. Co Nb, W Ca, REM, Mg, B (*1) (*2) (*3) Remarks A 0.23 ——   0.6 −0.438 −0.153 Compliant Example B 0.09 — — −1.1 −0.354 −0.146Compliant Example C 0.35 — — −1.8 −0.426 −0.149 Compliant Example D 0.17Nb: 0.04 — −4.2 −0.428 −0.169 Compliant Example E 0.29 W: 0.31 — −9.6−0.411 −0.147 Compliant Example F 0.08 — Ca: 0.003 −4.4 −0.341 −0.104Compliant Example G 0.41 — Ca: 0.002, 15.4 −0.292 −0.132 CompliantExample REM: 0.002 H 0.32 — Mg: 0.003   5.2 −0.334 −0.147 CompliantExample I 0.38 — B: 0.002 −0.3 −0.385 −0.126 Compliant Example J 0.21Nb: 0.02 Ca: 0.002 14.4 −0.283 −0.121 Compliant Example K 0.33 — — −0.4−0.457 −0.149 Comparative Example L 0.12 — — −3.2 −0.346 −0.098Comparative Example M 0.41 — —   5.2 −0.347 −0.114 Comparative Example N0.16 Nb: 0.02 — −5.4 −0.266 −0.132 Comparative Example O 0.38 — —   1.4−0.389 −0.109 Comparative Example P 0.17 Nb: 0.04 —   1.1 −0.442 −0.090Comparative Example Q 1.09 — — −4.3 −0.361 −0.088 Comparative Example R0.31 Nb: 0.02, — 50.0 −0.401 −0.043 Comparative Example W: 0.56 S 0.24 —— −36.2  −0.380 −0.301 Comparative Example T 0.41 — — 22.6 −0.117 −0.129Comparative Example U 0.15 Nb: 0.04, — −7.7 −0.669 −0.220 ComparativeExample W: 0.87 V 0.42 — — 33.8 −0.253   0.106 Comparative Example W0.04 W: 0.91 — −16.5  −0.586 −0.408 Comparative Example * Underlinemeans outside the range of the invention * The remainder is Fe andincidental impurities (*1) Formula (1): −109.37C + 7.307Mn + 6.399Cr +6.329Cu + 11.343Ni − 13.529Mo + 1.276W + 2.925Nb + 196.775N − 2.621Ti −120.307 (*2) Formula (2): −0.0278Mn + 0.0892Cr + 0.00567Ni + 0.153Mo −0.0219W − 1.984N + 0.208Ti − 1.83 (*3) Formula (3): −1.324C + 0.0533Mn +0.0268Cr + 0.0893Cu + 0.00526Ni + 0.0222Mo − 0.0132W − 0.473N − 0.5Ti −0.514

TABLE 2 Quenching Tempering Steel Ac₃ Heating Holding Ac₁ HeatingHolding Pipe Steel point temp. time Cooling stop point temp. time No.No. (° C.) (° C.) (min) Cooling method temp. (° C.) (° C.) (° C.) (min)1 A 745 920 20 Air cooling 25 640 610 60 2 B 735 920 20 Air cooling 25650 615 60 3 C 755 920 20 Air cooling 25 645 615 60 4 D 745 920 20 Aircooling 25 645 595 60 5 E 715 810 20 Water cooling 25 655 590 60 6 F 705810 20 Air cooling 25 640 590 60 7 G 775 920 20 Water cooling 25 665 62060 8 H 765 920 20 Water cooling 25 660 610 60 9 I 745 900 20 Air cooling25 655 585 60 10 J 770 920 20 Air cooling 25 660 600 60 11 A 745 730 20Air cooling 25 640 610 60 12 B 735 920 20 Air cooling 25 650 660 60 13 K760 920 20 Air cooling 25 640 600 60 14 L 715 920 20 Water cooling 25645 615 60 15 M 750 810 20 Air cooling 25 650 615 60 16 N 735 900 20 Aircooling 25 665 620 60 17 O 755 920 20 Air cooling 25 645 595 60 18 P 745810 20 Water cooling 25 640 610 60 19 Q 760 920 20 Air cooling 25 655590 60 20 R 760 920 20 Air cooling 25 660 585 60 21 S 715 920 20 Aircooling 25 645 600 60 22 T 765 920 20 Water cooling 25 660 600 60 23 U755 920 20 Air cooling 25 635 585 60 24 V 760 920 20 Air cooling 25 655610 60 25 W 725 920 20 Air cooling 25 640 585 60 Tensile Micro-properties SSC resistance test structure Yield Tensile Presence orabsence of Steel Retained γ stress stress cracking Pipe (*1) YS TS TestTest No. (volume %) (MPa) (MPa) environment 1 environment 2 Remarks 12.9 808 848 Absent Absent Present Example 2 0.5 786 825 Absent AbsentPresent Example 3 0.5 796 837 Absent Absent Present Example 4 0.3 786831 Absent Absent Present Example 5 0.2 769 809 Absent Absent PresentExample 6 0.4 779 819 Absent Absent Present Example 7 18.4 798 834Absent Absent Present Example 8 8.1 812 855 Absent Absent PresentExample 9 0.5 778 825 Absent Absent Present Example 10 19.3 784 834Absent Absent Present Example 11 5.9 765 848 Present Present ComparativeExample 12 4.7 761 825 Present Present Comparative Example 13 0.6 804846 Present Present Comparative Example 14 1.2 771 825 Present PresentComparative Example 15 0.3 768 801 Present Present Comparative Example16 10.4 812 861 Present Present Comparative Example 17 0.4 783 829Present Present Comparative Example 18 0.6 788 831 Present PresentComparative Example 19 3.5 793 846 Present Present Comparative Example20 8.6 853 885 Present Present Comparative Example 21 1.3 787 835Present Present Comparative Example 22 20.4 769 887 Present PresentComparative Example 23 9.3 786 833 Present Present Comparative Example24 0.5 806 865 Present Present Comparative Example 25 18.7 784 842Present Present Comparative Example (*1) Retained γ: Retainedaustenite * Underline means outside the range of the invention

The steel pipes of the present examples all had high strength with ayield stress of 758 MPa or more, demonstrating that the steel pipes weremartensitic stainless steel seamless pipes having excellent SSCresistance that do not crack even when placed under a stress in aH₂S-containing environment. On the other hand, in Comparative Examplesoutside the range of the present invention, the steel pipes did not havedesirable SSC resistance, even though the desired high strength wasobtained.

1. A martensitic stainless steel seamless pipe for oil country tubulargoods having a yield stress of 758 MPa or more, the martensiticstainless steel seamless pipe having a composition that comprises, inmass %, C: 0.010% or more, Si: 0.5% or less, Mn: 0.05 to 0.24%, P:0.030% or less, S: 0.005% or less, Ni: 4.6 to 8.0%, Cr: 10.0 to 14.0%,Mo: 1.0 to 2.7%, Al: 0.1% or less, V: 0.005 to 0.2%, N: 0.1% or less,Ti: 0.06 to 0.25%, Cu: 0.01 to 1.0%, and Co: 0.01 to 1.0%, in which thevalues of the following formulae (1), (2), and (3) satisfy the formulae(4) below, and the balance is Fe and incidental impurities:−109.37C+7.307Mn+6.399Cr+6.329Cu+11.343Ni−13.529Mo+1.276W+2.925Nb+196.775N−2.621Ti−120.307,  Formula(1)−0.0278Mn+0.0892Cr+0.00567Ni+0.153Mo−0.0219W−1.984N+0.208Ti−1.83,  Formula(2)−1.324C+0.0533Mn+0.0268Cr+0.0893Cu+0.00526Ni+0.0222Mo−0.0132W−0.473N−0.5Ti−0.514,  Formula(3) wherein C, Mn, Cr, Cu, Ni, Mo, W, Nb, N, and Ti represent thecontent of each element in mass %, and the content is 0 (zero) forelements that are not contained,−35.0≤value of formula (1)≤45.0,−0.600≤value of formula (2)≤−0.250, and−0.400≤value of formula (3)≤0.100.  Formulae (4)
 2. The martensiticstainless steel seamless pipe for oil country tubular goods according toclaim 1, wherein the composition further comprises, in mass %, one ortwo selected from Nb: 0.1% or less, and W: 1.0% or less.
 3. Themartensitic stainless steel seamless pipe for oil country tubular goodsaccording to claim 1, wherein the composition further comprises, in mass%, one, two or more selected from Ca: 0.010% or less, REM: 0.010% orless, Mg: 0.010% or less, and B: 0.010% or less.
 4. A method formanufacturing a martensitic stainless steel seamless pipe for oilcountry tubular goods, the method comprising: forming a steel pipe froma steel pipe material of the composition of claim 1; quenching the steelpipe by heating the steel pipe to a temperature equal to or greater thanan Ac₃ transformation point, and cooling the steel pipe to a coolingstop temperature of 100° C. or less; and tempering the steel pipe at atemperature equal to or less than an Ac₁ transformation point.
 5. Themartensitic stainless steel seamless pipe for oil country tubular goodsaccording to claim 2, wherein the composition further comprises, in mass%, one, two or more selected from Ca: 0.010% or less, REM: 0.010% orless, Mg: 0.010% or less, and B: 0.010% or less.
 6. A method formanufacturing a martensitic stainless steel seamless pipe for oilcountry tubular goods, the method comprising: forming a steel pipe froma steel pipe material of the composition of claim 2; quenching the steelpipe by heating the steel pipe to a temperature equal to or greater thanan Ac₃ transformation point, and cooling the steel pipe to a coolingstop temperature of 100° C. or less; and tempering the steel pipe at atemperature equal to or less than an Ac₁ transformation point.
 7. Amethod for manufacturing a martensitic stainless steel seamless pipe foroil country tubular goods, the method comprising: forming a steel pipefrom a steel pipe material of the composition of claim 3; quenching thesteel pipe by heating the steel pipe to a temperature equal to orgreater than an Ac₃ transformation point, and cooling the steel pipe toa cooling stop temperature of 100° C. or less; and tempering the steelpipe at a temperature equal to or less than an Ac₁ transformation point.8. A method for manufacturing a martensitic stainless steel seamlesspipe for oil country tubular goods, the method comprising: forming asteel pipe from a steel pipe material of the composition of claim 5;quenching the steel pipe by heating the steel pipe to a temperatureequal to or greater than an Ac₃ transformation point, and cooling thesteel pipe to a cooling stop temperature of 100° C. or less; andtempering the steel pipe at a temperature equal to or less than an Ac₁transformation point.